When calcium dissolved in the bloodstream is allowed to enter the muscle cells in arterial walls through special channels or pores in the cell membrane, it triggers events that cause the cell to shorten and the arteries to constrict or narrow. When this happens, the total capacity of the arteries is reduced, and if the total volume of blood remains constant, the blood pressure rises.

The calcium channel blockers prevent (block) the calcium from entering the cells. When the muscles of the arteries are prevented from constricting, blood vessels open up, allowing blood to flow more easily through them. This, in turn, reduces the blood pressure.

In rare cases, calcium channel blockers can cause heart block, rapid beating of the heart, or heart failure. The patient should be monitored very carefully.

In a study published in the 1995 issue of the Journal of the American Medical Association, the use of short-acting calcium channel blockers, especially in high doses, was found to be associated with a 60 percent increase in risk for heart attacks.

National Institutes of Health issued a warning that stated that short- acting nifedipine should be used "with great caution, if at all" and that the safety of related drugs was unclear.